Impaired hearing is today a very common handicap. It is above all, elderly people and people who are exposed to loud noise, that are affected. We do not discuss the causes of impairments in detail here, but only note that today it is practically impossible to treat these impairments in a medical way. The most common method today to re-establish, at least partly, the hearing of the affected patient, is to let the patient use some type of hearing aid. High demands must be put on such hearing aids, i.e. their frequency response must be adjusted to the patients hearing deficiency and it must also be possible to amplify desired sounds as for example normal conversation. To suit all normally occurring environmental situations it is not unusual that the same patient today has two or more hearing aids, which he or she alters between. The hearing aids must also be small and convenient to use.
Today there exist about a hundred different types of hearing aids on the market and it is therefore difficult for the person responsible for the fitting to decide which one is optimal in the individual case.
An estimate is that one out of four hearing aids is not acceptable by the patient and therfore the hearing aid is not used. As about 2.3 million hearing aids (1980) are distributed in the world every year, there is a great need for improving the devices and to develop more accurate and simplified fitting methods.
It would also be desirable to reduce the number of hearing aid types on the market to a few main types on condition that these main types can be adapted to each individual need.
Different types of filters with variable frequency response are earlier disclosed in the patent literature. Such filters are for example disclosed in the U.S. Pat. No. 3,989,904 filed Dec. 30, 1974, with the title "Method and apparatus for setting an aural prosthetis to provide specific auditory deficiency corrections", and in the Danish Patent Publication No. 138.149, filed Feb. 23, 1973, with the title "Kobling til brug i et horeapparat og i et apparat til m.ang.ling af menneskelige horedefekter".
The American invention refers to a device intended for adjusting a hearing aid in such a way that the gain in different frequency bands and maximum power output can be adjusted at the fitting procedure. The device has a number of disadvantages. For example the hearing aid can be optimal in adjusted for only one sound environment.
The Danish invention refers to a similar device where every filter individually can be adjusted with respect to the amplification. In this invention also only one frequency response can be set and the patient can hear well or optimally in just one sound environment, for example at normal conversations at home, while the device can be practically impossible to use in other sound environments, such as for example at place of work with disturbing background noises, traffic environment or at meetings, parties and the like.
In the U.S. Pat. No. 4,187,413 is further disclosed a hearing aid which includes a memory multiplexer for loading of multiplier coefficients for adapting the transfer function to different types of hearing deficiency. The hearing aid is possible to reprogram without disassembly. The programmed parameters are however related to one present hearing deficiency and not to various listening situations which can occur. I.e. only one signal process can be programmed at one time. There are therefor not any possibility to alter between a number of different signal processes suitable for various sound environments.